Migrate away from bintray

[wilsoniya]

Looks like bintray no longer exists, yet is a source of dependencies for this project: https://github.com/airbnb/kafka-statsd-metrics2/blob/master/build.gradle#L15

Additionally, bintray is also mentioned in the README.

[seizethedave]

Aye, this breaks the local build.

davidgrant@local kafka-statsd-metrics2 % ./gradlew shadowJar
> Task :compileJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileJava'.
> Could not resolve all files for configuration ':compileClasspath'.
   > Could not resolve org.slf4j:slf4j-log4j12:+.
     Required by:
         project :
      > Failed to list versions for org.slf4j:slf4j-log4j12.
         > Unable to load Maven meta-data from http://dl.bintray.com/airbnb/jars/org/slf4j/slf4j-log4j12/maven-metadata.xml.
            > Could not get resource 'http://dl.bintray.com/airbnb/jars/org/slf4j/slf4j-log4j12/maven-metadata.xml'.
               > Could not GET 'http://dl.bintray.com/airbnb/jars/org/slf4j/slf4j-log4j12/maven-metadata.xml'. Received status code 502 from server: Bad Gateway
[...]
BUILD FAILED in 5s
1 actionable task: 1 executed

Commenting out that bintray line in build.gradle seems to fix it.